Gallium nitride materials include gallium nitride and its alloys such as aluminum gallium nitride, indium gallium nitride and aluminum indium gallium nitride. These materials are semiconductor compounds that have a relatively wide, direct bandgap, which permits highly energetic electronic transitions to occur. Gallium nitride materials have a number of attractive properties including high electron mobility, the ability to efficiently emit blue light and the ability to transmit signals at high frequency, among others. Accordingly, gallium nitride materials are being investigated in many microelectronic applications such as transistors and optoelectronic devices.
Despite the attractive properties noted above, a number of challenges exist in connection with developing gallium nitride material-based devices. For example, it may be difficult to grow high quality gallium nitride materials on certain substrates, particularly silicon, due to property difference (e.g., lattice constant and thermal expansion coefficient) between the gallium nitride material and the substrate material. Also, it has been challenging to form gallium nitride material devices meeting the cost requirements for certain applications.
High power and medium power gallium nitride microwave transistors are now available and all types use a multifinger structure. Some of the power switching devices described in the research literature also use multifinger structures. Alternative new matrix island based structures are shown herein and these confer significant advantages in all switching applications. Following the practice of all power transistors, the structures are optimized for grounded source circuit applications where it is desirable to minimize the inductance and resistance of the source connection. To this end the transistors are commonly constructed with a series of via connections that subtend the entire vertical structure. These commonly used through-substrate via connections are difficult to manufacture and control. To reach the areas where smaller number of large vias can be made, air bridges may have to be constructed from each of the source connections. See, for example, U.S. Pat. No. 7,352,016 B2. However, air bridges are a source of manufacturing and handling problems.
U.S. Pat. No. 7,550,821 B2 (Shibata et al.) discloses a nitride semiconductor device in which air bridges are eliminated altogether. A plurality of first electrodes and a plurality of second electrodes are formed (spaced apart from each other) on an active region in a nitride semiconductor layer (which is formed on a main surface of a substrate). An interlayer insulating film is formed on the nitride semiconductor layer. The interlayer insulating film has openings that respectively expose the first electrodes and has a planarized top surface. A first electrode pad is formed in a region over the active region in the interlayer insulating film and is electrically connected to the exposed first electrodes through the respective openings. While the source-substrate contacts (short vias) are placed adjacent to the active areas and are directly connected to the source electrodes, there is an area increase penalty in this multifinger structure. As such, the nitride semiconductor device of Shibata et al. is also accordingly limited by the high on-resistance typical of power switching transistors using conventional multifinger structures.
U.S. Pat. No. 7,250,641 B2 (Saito et al.) discloses a nitride semiconductor device that comprises: a silicon substrate; a first aluminum gallium nitride layer formed as a channel layer on the silicon substrate in an island shape; and a second aluminum gallium nitride layer formed as a barrier layer of a first conductive type or i-type on the first aluminum gallium nitride layer. The islands disclosed therein are completely isolated from each other with no common gate electrode between them; each island is thus a separate device. The embodiments disclosed by Saito et al (e.g. as in FIG. 1) require the juxtaposition of three source electrodes, island to island. The island concept disclosed by Saito et al, serves only as a separation of isolated devices; i.e. there is no intrinsic mode of operation invoked between the islands.
The new topology described herein eliminates source connection air bridges and allows the gate electrode to be tracked in up to two additional directions leading to an on-resistance reduction of 1.5 to 5 times compared with conventional multi-finger structures. In this way the large area requirements of ladder (or multifinger structures) are eliminated.
A few examples of the present invention may be based upon relatively complex silicon based templates. However this, together with the new island based surface topologies, greatly simplify the costly gallium nitride device process steps.